1. Field of the Invention
This invention relates to alkoxysilane-terminated, moisture-hardening polyurethanes, to a process for their production, and to their use in adhesives and sealing compounds.
2. Statement of Related Art
Moisture-hardening one-component polyurethane systems containing free NCO groups are known as a basis for paints, adhesives and sealing compounds. In other known systems, NCO groups of a polyurethane prepolymer are reacted with alkoxysilanes in which an allyl group on silicon carries an isocyanate-reactive group (for example mercapto, amine), see, e.g. U.S. Pat. No. 3,627,722. This technique is said to improve adhesion to various inorganic surfaces. However, hardening (crosslinking) always involves the participation of unreacted, free NCO groups.
Polyurethanes reacted with organofunctional alkoxysilanes are also a constituent of many primers on inorganic materials. In this case, too, the Si(OR).sub.3 groups improve adhesion between the inorganic substrate and the actual paint. In addition, published German application No. 25 51 275 describes an automobile sealing compound which hardens solely through terminal alkoxysilane groups. Extremely hard compounds, for example having a shore-A-hardness of 63, are obtained.
One-component polyurethane systems which harden substantially through terminal isocyanate groups occasionally give off small bubbles in the hardened mass from carbon dioxide partially released during the crosslinking process. By contrast, hardening through Si(OR).sub.3 groups is in principle bubble-free.
In their hardened state, however, polyurethane systems containing terminal Si(OR).sub.3 groups generally form hard, brittle compositions of limited elasticity. In this form, they are not suitable as a basis for sealing compounds because for many applications soft and elastic materials of high resilience are required.
Accordingly, systems of the above type having greater elasticity than, for example, the polymer systems described in German application No. 25 51 275 would be desirable.